With the requirements for scale-down of a semiconductor element, low power consumption of a semiconductor device and improvement in the operation speed of a semiconductor device, the threshold voltage of a transistor in a semiconductor device has been lowered. However, because of a low threshold voltage, there arises a problem that it is difficult to determine if a true fault is generated on the occasion of inspecting whether a leak current is generated between the source and drain of a transistor of the semiconductor device. Moreover, a problem generated when a semiconductor device is in the condition for waiting operation is that the power consumption increases with a leak current of transistor. In order to overcome such problem, a technology has been proposed in which the threshold voltage of transistor is temporarily raised to reduce a leak current by applying a predetermined voltage to the semiconductor substrate where transistors are arranged (in more practical terms, semiconductor region called a well). Thereby, for example, during the testing period, when the transistor is turned ON even if the threshold value is raised, such transistor can be determined as a defective one. Moreover, when the semiconductor device is in the condition for waiting operation, a leak current can be reduced and power consumption of the semiconductor device can also be lowered through increase of the threshold voltage of transistor. The technique for varying this threshold voltage is described, for example, in the NIKKEI MICRODEVICE (August, 1996), pp. 50–66 (issued on Aug. 1, 1996), NIKKEI BP, Inc. Namely, a circuit structure and an element layout structure are disclosed for the technique to change the threshold voltage through the feedback control of substrate voltage and to realize low power consumption and high-speed operation.
Here, the inventors of the present invention have found, as a result of discussion, that there still exists following problem in the technique that the threshold voltage can be varied using a switch element for using the power supply voltage or the other voltage as the substrate (or well) voltage.
Namely, in this technique, a switch element for changing over the power supply voltage or the other voltage to be used as the substrate (well) is required, but sufficient consideration is necessary for allocation of such switch element and if an excessive amount of switch elements are used, the allocation area of the logic circuits becomes small. Therefore, a chip size increases. Meanwhile, if the number of such switch elements is too small, noise is generated in the well resulting from increase of resistance in the well. Therefore, the threshold voltage varies and operation of semiconductor device becomes unstable. Moreover, such noise results in the problem that latch-up is easily induced in the semiconductor device having the CMIS circuit.
It is therefore an object of the present invention to provide a technique to reduce the noise generated at the semiconductor region in a semiconductor device having the function to control the threshold voltage of a transistor through the ON/OFF control of a switch element provided between the semiconductor region where transistors are formed and the power supply wiring.
Moreover, it is also an object of the present invention to provide a technique to reduce the noise generated at the semiconductor region without increase in size of the semiconductor device having the function to control the threshold voltage of a transistor through ON/OFF control of a switch element provided between the semiconductor region where transistors are formed and the power supply wiring.
In addition, it is also an object of the present invention to provide a technique to reduce noise generated at the semiconductor region without complication of a structure of the semiconductor device having the function to control the threshold voltage of a transistor through ON/OFF control of a switch element provided between the semiconductor region where transistors are formed and the power supply wiring.
The aforementioned and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.